Organic electroluminescent device with continuous organic medium

ABSTRACT

An organic EL device includes a cathode, an organic electroluminescent layer and an anode, laminated in sequence. The organic layer is composed of a continuous organic medium A x  B y , where A and B are components capable of transporting electrons and holes, respectively, x represents the content of the A component with a value ranging from 0 adjacent the anode to 100% adjacent the cathode, and y represents the content of the B component with a value ranging from 0 adjacent the cathode to 100% adjacent the anode. The lifetime of the device is improved by the elimination of heterojunctions in the continuous organic medium and suppression of the re-crystallization tendency of the organic materials. At least one fluorescent dye or pigment is incorporated in the organic layer to improve efficiency and color of the luminescence.

FIELD OF THE INVENTION

This invention relates to an organic electroluminescent (EL) device andmore specifically to, the organic medium in the organic EL device.

BACKGROUND OF THE INVENTION

Organic electroluminescent (EL) devices are generally composed of threelayers of organic materials sandwiched between transparent and metallicelectrodes, the three layers including an electron transporting layer,an emissive layer and a hole transporting layer. Organic EL devices areattractive owing to the requirement for low driving voltage and thepotential application to full color flat emissive displays. Thoughsignificant lifetime has been achieved in the prior art (See U.S. Pat.No. 4,720,432), further improvement is needed for applications wherehigh brightness is required. Among other things, the lifetime of anorganic EL device is affected by the stability of both the bulkmorphology of the hole transporting materials and the interface betweenthe hole and electron transporting layers when the organic EL device isunder bias.

Several schemes have been proposed to address the problem of bulkmorphology stability of the hole transporting materials in an organic ELdevice: one being a double-layer hole transporting configuration (seeU.S. Pat. No. 5,256,945); and another being usage of materials of highglass transition temperatures (U.S. Pat. No. 5,061,569).

It is a purpose of this invention to address the problem of theinterface stability between heterojunctions in an organic EL device.

It is another purpose of this invention to provide a new method toenhance the thermal stability of the organic electroluminescent mediumof an organic EL device.

It is a further purpose of this invention to provide an organicelectroluminescent device for displays with improved reliability.

SUMMARY OF THE INVENTION

The above problems and others are at least partially solved and theabove purposes and others are realized in an organic EL device includinga cathode, an organic electroluminescent layer and an anode, laminatedin sequence, wherein said organic electroluminescent layer is composedof a continuous organic medium without heterojunctions.

According to the present invention, there is obtained an organic ELdevice with an improved reliability by eliminating the heterojunctions,and suppressing the aggregation or re-crystallization tendency oforganic materials with the formation of a single layer of an organicalloy.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a simplified sectional view of an organicelectroluminescent device in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the specification concludes with claims defining the features ofthe invention that are regarded as novel, it is believed that theinvention will be better understood from a consideration of thefollowing description in conjunction with the single drawing FIGURE.

Referring now to single FIG. 1, there is illustrated an organicelectroluminescent device 10 in accordance with the present invention.Device 10 is fabricated upon a first substrate 12 which is transparentand may be fabricated of any of the number of known materials employedin the art. For example, substrate 12 may be fabricated of a glass, suchas a Corning 7059 glass, transparent plastic substrates made ofpolyolefins, polyethersulfones, polyarylates. In one preferredembodiment, substrate 12 is fabricated of glass of quality good for flatpanel display applications.

Deposited atop substrate 12 is a first electrode 14, which iselectrically conductive and optically transparent or semi-transparent.Several materials may be advantageously employed as first electrode 14for an OED device. Examples of materials include conductive metal oxidessuch as indium oxide, indium-tin oxide (ITO), zinc oxide, zinc-tinoxide, conductive transparent polymers such as polyaniline.Alternatively, electrode 14 may be fabricated of a semi-transparentmetal, examples of which include a thin layer (<500 Å) of gold, copper,silver, and combinations thereof. In one preferred embodiment, electrode14 is fabricated of ITO or zinc oxide.

Thereafter, atop the first electrode 14 is deposited a layer 16 of whatis known as the hole injecting material that also acts as a buffer layerto match the thermal and mechanical properties of the first electrodeand the subsequent layers of organic materials. Hole injecting layer 16is preferably comprised of a porphyrinic compound of the type disclosedby Adler in U.S. Pat. No. 3,935,031 or Tang in U.S. Pat. No. 4,356,429.Examples of the preferred compounds include copper phthalocyanine, zincphthalocyanine. Alternative hole injecting materials such as carbon orcarbon nitride films can also be used. It is to be understood that holeinjecting layer 16 is optional, and need not necessarily be provided.

Thereafter deposited atop hole injecting layer 16 (if present) is anorganic electroluminescent medium layer 18 where light emission takesplace. The composition of organic electroluminescent medium layer 18 isone of the key subjects of the present invention and will be describedin greater detail herein below.

Thereafter deposited atop organic electroluminescent medium layer 18 isan electron injecting layer 20. Electron injecting layer 20 is used tofacilitate the injection of electrons from a cathode (yet to be formedon top of layer 20) to organic electroluminescent medium layer 18. Ingeneral, electron injection layer 20 is entirely optional, and may beomitted without significant compromise of the device performance.

Deposited atop electron injecting layer 20 (if present) is a secondelectrode 22 (cathode) which is typically formed of a metal with a workfunction of less than 4 eV and at least one other protective metal ofhigher work function. The preferred low work function metal is selectedfrom a group of lithium, magnesium, calcium, or strontium, while thepreferred high work function metal is selected from a group of aluminum,indium, copper, gold, or silver. Second electrode 22 can also be formedof an alloy of a lower work function metal and a high work functionmetal by coevaporation. The content of the low work function metal inthe second electrode can vary from 0.1% to 50%, but preferably below20%. Alternatively, second electrode 22 can be a metal of work functionof greater than 4 eV such as aluminum, silver etc. when electroninjection layer 20 is composed of alkaline fluoride such as LiF, MgF₂ oralkaline oxide such as LiO_(x), MgO_(x), CaO_(x), CsO_(x).

Throughout this disclosure, because both layers 16 and 20 are optional,these layers will be considered as a portion of the anode and cathode,respectively. Thus, in this described embodiment of an organic EL devicein accordance with the present invention an anode including layers 14and 16, a single organic electroluminescent layer 18 and a cathodeincluding layers 20 and 22 are laminated in sequence on substrate 12.Also, while the preferred embodiment includes a transparent substrateand anode, it will be understood by those skilled in the art that theentire structure could be reversed so that the light is emitted upwardlyin FIG. 1 and the substrate could then be an opaque material.

When a potential is applied between first and second electrodes 14 and22 by means of a potential source 17, holes are injected from firstelectrode 14 and hole injecting layer 16 (if present) and electrons areinjected from second electrode 22 and electron injecting layer 20 (ifpresent) into organic electroluminescent medium layer 18, wherein uponan electron and a hole recombination a photon is emitted.

The instant disclosure is directed to the organic electroluminescentmedium in layer 18. In this embodiment, the organic electroluminescentmedium (layer 18) is composed of a continuous organic medium withoutheterojunctions. The continuous organic medium has a general formula ofA_(x) B_(y), where A is a component capable of transporting electrons, Bis a component capable of transporting holes, x represents the contentof the A component in the A_(x) B_(y) medium with a value ranging from 0adjacent the anode to 100% adjacent the cathode, and y represents thecontent of the B component in the A_(x) B_(y) medium with a valueranging from 0 adjacent the cathode to 100% adjacent the anode. Therelationship between x and y is: x=100%-y. While the variations of X andY across layer 18 will generally be substantially linear, it will beunderstood by those skilled in the art that variations other than linearmay be incorporated in specific applications.

In this embodiment, it is preferred that in the direction from the anodeto the cathode in the A_(x) B_(y) medium (layer 18) the content of the Acomponent (x) gradually increases while the content of the B component(y) gradually decreases. One extreme case will be that the content ofthe A component increases from 0% to 100%, meantime the content of the Bcomponent decreases from 100% to 0% in the direction from layer 16 tolayer 20. Of course, it is also possible to start A_(x) B_(y) with xbeing a fraction of a percent (e.g. x=0.1%, y=99.9%) and end up with ybeing a fraction of a percent (e.g. x=99.9%, y=0.1%). The rates ofincrease of the A component and decrease of the B component can becontrolled by relative evaporation or sputtering rates of the A and Bcomponents.

Though component A in the A_(x) B_(y) medium can be any one of theelectron transporting materials known to those skilled in the art, it ispreferred to be selected from the group of organometallic complexesdisclosed in U.S. Pat. Nos. 4,769,292 and 5,529,853, and in a pendingU.S. patent application entitled "NEW ORGANOMETALLIC COMPLEXES FOR USEIN LIGHT EMITTING DEVICES", filed Sep. 12, 1994, bearing Ser. No.08/304,451, (now abandoned) and assigned to the same assignee.Characteristics of an appropriate electron transporting material (A) inaccordance with the present invention include tris(8-quinolinol)aluminum, bis(10-oxo-benzo[h] quinoline beryllium,bis(2-(2-oxy-phenyl)benzoxazole) zinc,bis(2-(2-oxy-phenyl)benzothiazole) zinc, Bis(2-methyl-8-quinolinolato)aluminum-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum, Bis(2-methyl-8-quinolinolato)(ortho-cresolato)aluminum, or a combination ofthe above materials.

The hole transporting component B includes organic tertiary aromaticamines having a general structure of ##STR1## where: Ar₁, Ar₂ and Ar₃are independently aromatic hydrocarbons or aromatic tertiary aminemoieties. The aromatic hydrocarbons and the aromatic tertiary aminemoieties in turn can be substituted. Typical substituents includes alkylgroups, alkoxy groups, alkylamine groups, aryl groups, aryloxy groups,arylamine groups and halogen such as bromide, chloride, and fluoride.

The following is a partial list of a few classes of aromatic tertiaryamines satisfying the requirement of the present invention for thecomponent B: ##STR2## Where R₁, R₂, R₃, R₄ and R₅ are independentlyselected from alkyl groups, alkoxy groups, alkylamine groups, arylgroups, aryloxy groups, arylamine groups and halogen such as bromide,chloride, and fluoride; and R₁, R₂, R₃, R₄ and R₅ may also beindependently a fused aromatic ring.

Though at the present time, the most reliable organic EL devices arefabricated mainly with the previously discussed aromatic tertiary aminesas hole transporting materials, other organic hole transportingmaterials such as aromatic silanes, aromatic silazanes and aromaticphosphine can also be used in the present invention as the B componentas long as those materials have a T_(g) greater than 75° C.: ##STR3##where: Ar₁, Ar₂, Ar₃, Ar₄, Ar₅, and Ar₆ are independently aromatichydrocarbons or aromatic tertiary amine moieties. The aromatichydrocarbons and the aromatic tertiary amine moieties in turn can besubstituted. Typical substituents include alkyl groups, alkoxy groups,alkylamine groups, aryl groups, aryloxy groups, arylamine groups andhalogen such as bromide, chloride, and fluoride.

Another embodiment of the present invention is directed toward theimprovement of the organic EL efficiency as well as control of emissioncolor. The purpose can be achieved by incorporation of a small fractionof a dye or pigment of high fluorescent efficiency into the whole orpart of organic electroluminescent layer 18. For efficient energytransfer, it is preferred that the fluorescent dye or pigment has abandgap no greater than that of the host material making up the organicelectroluminescent medium (A_(x) B_(y)). It is also preferred that thefluorescent dye or pigment is present in a concentration of from 10⁻³ to10 mole percent, based on the moles of the material comprised of theA_(x) B_(y) medium. The selection of a fluorescent dye or pigmentsuitable for organic EL applications is well known to those skilled inthe art and the same principles may generally be applied in the presentembodiment.

Thus an organic EL device with a continuous organic electroluminescentmedium without heterojunctions is obtained. The lifetime of the organicEL device is improved, by the elimination of heterojunctions in theorganic electroluminescent medium and suppression of the aggregation orre-crystallization tendency of organic materials, with the formation ofan organic alloy for the organic electroluminescent medium. The emissionzone of the organic EL device is controlled by adjusting where in thecontinuous organic electroluminescent medium a fluorescent dye orpigment is incorporated. The emission efficiency as well as color of theorganic EL device is controlled by the selection of the dye or pigment,and the concentration of the dye or pigment in the organicelectroluminescent medium.

While we have shown and described specific embodiments of the presentinvention, further modifications and improvements will occur to thoseskilled in the art. We desire it to be understood, therefore, that thisinvention is not limited to the particular forms shown and we intend inthe appended claims to cover all modifications that do not depart fromthe spirit and scope of this invention.

What is claimed is:
 1. An organic medium for use in anelectroluminescent display device comprising a layer of a continuousorganic electroluminescent medium with a formula of A_(x) B_(y) andhaving a thickness defined by a first edge and an oppositely opposedspaced apart second edge, where A is a component capable of transportingelectrons, B is a component capable of transporting holes, x representsthe content of the A component in the layer of organicelectroluminescent medium with a value ranging from 0 adjacent the firstedge of the layer of organic electroluminescent medium to 100% adjacentthe second edge of the layer of organic electroluminescent medium, and yrepresents the content of the B component in the layer of organicelectroluminescent medium with a value ranging from 0 adjacent thesecond edge of the layer of organic electroluminescent medium to 100%adjacent the first edge of the layer of organic electroluminescentmedium.
 2. An organic medium for use in an electroluminescent displaydevice as claimed in claim 1 wherein the A component is selected from agroup of organometallic complexes consisting of tris(8-quinolinol)aluminum, bis(10-oxo-benzo[h] quinoline beryllium,bis(2-(2-oxy-phenyl)benzoxazole) zinc,bis(2-(2-oxy-phenyl)benzothiazole) zinc,Bis(2-methyl-8-quinolinolato)aluminum-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum,and Bis(2-methyl-8-quinolinolato)(ortho-cresolato)aluminum, or acombination of the above organometallic complexes.
 3. An organic mediumfor use in an electroluminescent display device as claimed in claim 1wherein the B component is selected from a group of aromatic amineshaving a general structure of ##STR4## where: Ar₁, Ar₂ and Ar₃ areindependently aromatic hydrocarbons or aromatic tertiary amine moieties,or the aromatic hydrocarbons and the aromatic tertiary amine moietiesare in turn substituted with substituents selected from alkyl groups,alkoxy groups, alkylamine groups, aryl groups, aryloxy groups, arylaminegroups and halogen.
 4. An organic medium for use in anelectroluminescent display device as claimed in claim 3 wherein the Bcomponent is selected from a group of aromatic amines having thefollowing chemical formulas: ##STR5## where R₁, R₂, R₃, R₄ and R₅ areindependently selected from alkyl groups, alkoxy groups, alkylaminegroups, aryl groups, aryloxy groups, arylamine groups, halogen, and afused aromatic ring.
 5. An organic medium for use in anelectroluminescent display device as claimed in claim 1 wherein the Bcomponent is selected from organic aromatic silanes, aromatic silazanesand aromatic phosphine with general formulas: ##STR6## where: Ar₁, Ar₂,Ar₃, Ar₄, Ar₅, and Ar₆ are independently aromatic hydrocarbons oraromatic tertiary amine moieties or the aromatic hydrocarbons and thearomatic tertiary amine moieties are in turn substituted withsubstituents selected from alkyl groups, alkoxy groups, alkylaminegroups, aryl groups, aryloxy groups, arylamine groups and halogen, eachsilane, silazane or phosphine having a T_(g) greater than 75° C.
 6. Anorganic medium for use in an electroluminescent display device asclaimed in claim 1 including in addition at least one fluorescent dye orpigment incorporated in the layer of a continuous organic medium havinga formula of A_(x) B_(y).
 7. An organic medium for use in anelectroluminescent display device as claimed in claim 6 wherein thefluorescent dye or pigment is present in a concentration of from 10⁻³ to10 mole percent, based on moles of continuous organic medium A_(x)B_(y).
 8. An organic medium for use in an electroluminescent displaydevice as claimed in claim 6 wherein the organic electroluminescentmedium (A_(x) B_(y)) has a bandgap and the fluorescent dye or pigmenthas a bandgap no greater than the bandgap of the organicelectroluminescent medium (A_(x) B_(y)).
 9. An organic medium for use inan electroluminescent display device comprising an organicelectroluminescent layer having a thickness defined by a first side andan oppositely opposed spaced apart second side and formed of acontinuous organic medium having a formula of A_(x) B_(y), where A is acomponent capable of transporting electrons, B is a component capable oftransporting holes, x represents the content of the A component in themedium, and y represents the content of the B component in the medium,where x plus y equal 100% of the continuous organic medium, and x has avalue of a fraction of a percent adjacent the first side of the organicelectroluminescent layer and y has a value of a fraction of a percentadjacent the second side of the organic electroluminescent layer.
 10. Anorganic electroluminescent device comprising a cathode, an organicelectroluminescent layer and an anode, laminated in sequence, whereinsaid organic electroluminescent layer is composed of a continuousorganic medium having a formula of A_(x) B_(y), where A is a componentcapable of transporting electrons, B is a component capable oftransporting holes, x represents the content of the A component in themedium with a value ranging from 0 adjacent the anode to 100% adjacentthe cathode, and y represents the content of the B component in themedium with a value ranging from 0 adjacent the cathode to 100% adjacentthe anode.
 11. An organic electroluminescence device as claimed in claim10, wherein the A component in the organic electroluminescent layer isselected from a group of organometallic complexes consisting oftris(8-quinolinol) aluminum, bis(10-oxo-benzo[h] quinoline beryllium,bis(2-(2-oxy-phenyl)benzoxazole) zinc,bis(2-(2-oxy-phenyl)benzothiazole) zinc,Bis(2-methyl-8-quinolinolato)aluminum-μ-oxo-bis(2-methyl-8-quinolinolato)aluminum,and Bis(2-methyl-8-quinolinolato)(ortho-cresolato)aluminum, or acombination of the above organometallic complexes.
 12. An organicelectroluminescence device as claimed in claim 10, wherein the Bcomponent in the organic electroluminescent layer is selected fromorganic tertiary aromatic amines having a general structure of ##STR7##where: Ar₁, Ar₂ and Ar₃ are independently aromatic hydrocarbons oraromatic tertiary amine moieties, or the aromatic hydrocarbons and thearomatic tertiary amine moieties are in turn substituted withsubstituents selected from alkyl groups, alkoxy groups, alkylaminegroups, aryl groups, aryloxy groups, arylamine groups and halogen. 13.An organic electroluminescence device as claimed in claim 10 wherein theB component in the organic electroluminescent layer is selected fromorganic aromatic silanes, aromatic silazanes and aromatic phosphine withgeneral formulas: ##STR8## where: Ar₁, Ar₂, Ar₃, Ar₄, Ar₅, and Ar₆ areindependently aromatic hydrocarbons or aromatic tertiary amine moieties,or the aromatic hydrocarbons and the aromatic tertiary amine moietiesare substituted with substituents selected from alkyl groups, alkoxygroups, alkylamine groups, aryl groups, aryloxy groups, arylamine groupsand halogen, each silane, silazane or phosphine having a T_(g) greaterthan 75° C.
 14. An organic electroluminescence device as claimed inclaim 10 wherein the B component in the organic electroluminescent layeris selected from a group of aromatic amines having the followingchemical formulas: ##STR9## where R₁, R₂, R₃, R₄ and R₅ areindependently selected from alkyl groups, alkoxy groups, alkylaminegroups, aryl groups, aryloxy groups, arylamine groups, halogen, and afused aromatic ring.
 15. An organic electroluminescence device asclaimed in claim 10 including at least one fluorescent dye or pigmentincorporated in the continuous organic medium A_(x) B_(y) of the organicelectroluminescent layer, the fluorescent dye or pigment being presentin a concentration of from 10⁻³ to 10 mole percent based on moles of theorganic electroluminescent medium (A_(x) B_(y)), and the organicelectroluminescent medium (A_(x) B_(y)) having a bandgap and thefluorescent dye or pigment having a bandgap no greater than the bandgapof the organic electroluminescent medium (A_(x) B_(y)).
 16. An organicelectroluminescent device comprising a cathode, an organicelectroluminescent layer and an anode, laminated in sequence, whereinsaid organic electroluminescent layer is composed of a layer of acontinuous organic medium having a formula of A_(x) B_(y), where A is acomponent capable of transporting electrons, B is a component capable oftransporting holes, x represents the content of the A component in themedium with a value ranging from 0 adjacent the anode to 100% adjacentthe cathode, and y represents the content of B component in the mediumwith a value ranging from 0 adjacent the cathode to 100% adjacent theanode, and at least one fluorescent dye or pigment incorporated in thelayer of organic electroluminescent medium.
 17. An organicelectroluminescence device as claimed in claim 16 wherein thefluorescent dye or pigment is present in a concentration of from 10⁻³ to10 mole percent, based on moles of the organic electroluminescent mediumA_(x) B_(y).
 18. An organic electroluminescence device as claimed inclaim 16 wherein the organic electroluminescent medium (A_(x) B_(y)) hasa bandgap and the fluorescent dye or pigment has a bandgap no greaterthan the bandgap of the organic electroluminescent medium (A_(x) B_(y)).19. An organic electroluminescent device comprising a cathode, anorganic electroluminescent layer and an anode, laminated in sequence,said organic electroluminescent layer having a thickness defined by afirst side and an oppositely opposed spaced apart second side and beingformed of a continuous organic medium having a formula of A_(x) B_(y),where A is a component capable of transporting electrons, B is acomponent capable of transporting holes, x represents the content of theA component in the medium, and y represents the content of the Bcomponent in the medium and where x plus y equal 100% of the continuousorganic medium, and x has a value of a fraction of a percent adjacentthe first side of the organic electroluminescent layer and y has a valueof a fraction of a percent adjacent the second side of the organicelectroluminescent layer.